Adsorption geometry and self-assembling of chiral modifier (R)-(+)-1-(1-naphthylethylamine) on Pt(111)

Flexible plasmonic nanocavities: a universal platform for the identification of molecular orientations

The molecular orientation provides fundamental images to understand molecular behaviors in chemistry. Herein, we propose and demonstrate sandwich plasmonic nanocavities as a surface-selection ruler to illustrate the molecular orientations by surface-enhanced Raman spectroscopy (SERS). The field vector in the plasmonic nanocavity presents a transverse spinning feature under specific excitations, allowing the facile modulation of the field polarizations to selectively amplify the Raman modes of the target molecules. It does not require the knowledge of the Raman spectrum of a bare molecule as a standard and thus can be extended as a universal ruler for the identification of molecular orientations. We investigated the most widely used Raman probe, Rhodamine 6G (R6G) on the Au surface and tried to clarify the arguments about its orientations from our perspectives. The experimental results suggest concentration-dependent adsorption configurations of R6G: it adsorbs on Au primarily via an ethylamine group with the xanthene ring lying flatly on the metal surface at low concentrations, and the molecular orientation gradually changes from "flat" to "upright" with the increase of molecular concentrations.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Competing Reaction Pathways in Heterogeneously Catalyzed Hydrogenation of Allyl Cyanide: The Chemical Nature of Surface Species

We present a mechanistic study on the formation of an active ligand layer over Pd(111), turning the catalytic surface highly active and selective in partial hydrogenation of an α,β‐unsaturated aldehyde acrolein. Specifically, we investigate the chemical composition of a ligand layer consisting of allyl cyanide deposited on Pd(111) and its dynamic changes under the hydrogenation conditions. On pristine surface, allyl cyanide largely retains its chemical structure and forms a layer of molecular species with the CN bond oriented nearly parallel to the underlying metal. In the presence of hydrogen, the chemical composition of allyl cyanide strongly changes. At 100 K, allyl cyanide transforms to unsaturated imine species, containing the C=C and C=N double bonds. At increasing temperatures, these species undergo two competing reaction pathways. First, the C=C bond become hydrogenated and the stable N‐butylimine species are produced. In the competing pathway, the unsaturated imine reacts with hydrogen to fully hydrogenate the imine group and produce butylamine. The latter species are unstable under the hydrogenation reaction conditions and desorb from the surface, while the N‐butylimine adsorbates formed in the first reaction pathway remain adsorbed and act as an active ligand layer in selective hydrogenation of acrolein.